The veteran joined two other Germans in the last eight of the Stuttgart event with a 6-0 3-6 6-4 victory over the big-serving Latvian.

Haas cruised through the first set, but Gulbis bounced back in the second. In the decider, Haas broke serve to win the match as Gulbis slammed a forehand into the net.

"There are days like that, I could feel that he couldn't find his rhythm in the first set but I was also struggling. This was my first match after Wimbledon, switching from grass to clay," Haas said.

"In the second, he got some first serves in and they are difficult to return. But I also served well at the end."

Haas will next play fifth-seeded Italian Fabio Fognini, who overwhelmed Leonardo Mayer of Argentina 6-1 6-3.

Fourth-seed Benoit Paire of France won his second-round match by beating Russian Nikolay Davydenkoa 6-3 6-1, and will play Victor Hanescu of Romania in the quarter-final, who beat Brazil's Thomaz Bellucci 6-1 6-4.

Paire advanced to the quarter-finals in Stuttgart for the first time, having twice lost in the second round previously.